The present invention generally relates to signal modulation, and particularly relates to imparting signal modulations by driving multiple Radio Frequency (RF) power amplifiers with single or multi-level Pulse Width Modulation (PWM) input signals, and combining their output signals efficiently to construct a multi-level PWM RF signal.
In many fields, for example in third and higher generation base stations, bandwidth-optimized modulation schemes are used for transmitting information. Bandwidth-optimized modulation schemes require a non-constant envelope, and thus have a relatively high peak-to-average power ratio (PAR). Linear power amplifiers such as class AB amplifiers are typically used because they offer high linearity. However, class AB amplifiers must be driven with a high back-off from the maximum (saturated) output power to achieve good linearity across a wide operating range. RF power amplifiers, of what ever class, e.g. type AB, or so-called switched-mode amplifiers of type D, E, F, etc., only achieve high efficiency when operated close to their maximum saturated power level. Thus, backing-off a class AB amplifier results in lower transmitted power, and thus reduced overall power efficiency.
Other conventional signal modulation techniques exist for Radio Frequency (RF) applications. However, each of the techniques suffers from poor power efficiency, poor linearity, complexity or other limitations, especially when the signal bandwidth is large. For example, supply voltage regulation techniques have poor power efficiency if the voltage regulator must have a large bandwidth. Linearity is problematic for Doherty amplifiers. Out-phasing, where two equally sized power amplifier outputs with an appropriately designed phase relation are combined via a power combiner, suffers from constant power dissipation. Delta sigma modulators used in conjunction with a high-power output stage tend to be less efficient than their PWM counterparts.
A pulse width modulator has been used to drive an RF power amplifier to impart amplitude signal modulations based on the duty cycle of the PWM signal applied to the RF amplifier. A conventional class AB or switched-mode RF power amplifier is driven into saturation by an input RF PWM signal, consisting of on-off bursts of an RF carrier with constant amplitude. The average burst duration (duty cycle) is made proportional to the baseband signal amplitude by so-called PWM coding. A way of generating a PWM signal is to compare, with a comparator circuit, the baseband signal to a threshold signal in the form of a triangle waveform with frequency fs, typically on the order of 10 times higher than the bandwidth B of the baseband signal. The comparator output signal is a baseband PWM signal, which is multiplied with an RF carrier, to form the RF PWM signal. In the case the RF carrier is phase modulation, the baseband signal fed to the comparator is typically the envelope of a complex baseband signal. When the baseband signal is below the threshold signal, there is ideally no input signal to, or output signal from, the RF power amplifier. The amplified RF PWM signal is then passed through a band-pass filter with bandwidth Bf, where typically B<Bf<fs, to remove the out-of-band power associated with the PWM, retaining only the desired amplified modulation RF signal. The efficiency of the RF power amplifier is ideally independent of the burst duration, apart from losses related to turning the amplifier on and off, which relatively become more significant the shorter the burst.
However, at lower baseband power levels, the PWM burst duration decreases, increasing the fraction of out-of-band versus in-band spectral power in the RF PWM signal. If the band-pass filter is connected directly to the amplifier output, it will typically present impedance terminations for the out-of-band spectral components which will prevent the amplifier from operating efficiently, if special care is not taken in the design of the amplifier, possibly leading to a more complex implementation. A circulator can be inserted between the amplifier output and the band-pass filter input, causing the out-of-band power to be reflected to the circulator dump port and dissipated into a matched load, without upsetting the amplifier. This has the drawback of reduced overall transmitter efficiency for low baseband signal levels (corresponding to short bursts), where the out-of-band power is high, although the RF amplifier itself is still operating efficiently.